1. Field of the Invention.
This invention relates in general to optical signal processing, and more particularly to acoustooptic diffraction utilizing a modified multi-channel Bragg cell.
2. Description of the Prior Art
An important objective in the field of radar electronic support measures is the development of wide band radar receivers for real time sorting and identification of incoming simultaneous microwave signals.
One prior art approach to simultaneous signal sorting utilizes an interferometric acoustoopitc signal processor employing a multi-channel Bragg cell. Such a configuration is described in an article by the present inventor J. P. Y. Lee, entitled "Interferometric Acoustooptic Signal Processor For Simultaneous Direction Finding and Spectrum Analysis", Applied Optics, Vol. 22, No. 4, pages 867-872, Mar. 15, 1983.
According to this prior art configuration, an incoming microwave signal is received at a predetermined angle of incidence by a plurality of strategically placed antennae. The received incoming signal is applied to a multi-channel Bragg cell comprised of a plurality of transducers and a block of acoustooptic material. Each transducer is connected to a respective one of the antennae, and the transducer spacing is made proportional to the spacing of the respective antennae. The received signal is applied via the transducers to the block of acoustooptic material, resulting in an acoustic field or wave pattern being generated therein.
A laser beam is used to illuminate the acoustooptic block, and is diffracted due to interaction with the acoustic field generated therein. The diffracted laser beam is then focussed via a Fourier transform lens onto a back focal plane at a predetermined point, defined by Cartesian coordinates in relation to a point of orgin on the focal plane designative of an undiffracted beam. The intensity distribution of the Fourier transform represents the power spectrum of the signal on one axis, and the angle of incidence on the other axis.
This prior art approach has been found to be very useful for microwave signal sorting wherein the incoming signals are spatially separated in frequency and direction of arrival (i.e. angle of incidence). The direction of arrival and frequency resolution are obtained by combining the inherently high angular resolution of an interferometer with the frequency spectral analkysis capability of a Bragg cell receiver.
According to the aforementioned article of J. P. Y. Lee, it was concluded that an aperiodic array with four or five transducers constituted the optimum configuration for a broadband two-dimensional acoustooptic receiver. This conclusion was based on factors such as complexity, cost, required main lobe beamwidth, side lobe level and antenna size.
However, for this optimum configuration it was found that the inter-transducer spacing was required to be much larger than the space occupied by the acoustic transducers themselves. Thus, in the event that the prior art multi-channel Bragg cell is uniformly illuminated by the laser beam, a large portion of the incident light does not interact with the acoustic waves within the cell, even though there may be an appreciable spread in the acoustic wave profile. This has been found to reduce the amount of light diffracted within the Bragg, cell, resulting in a corresponding drop in sensitivity of the receiver.